Modern drilling techniques employ an increasing number of sensors in downhole tools to determine downhole conditions and parameters such as pressure, spatial orientation, temperature, gamma ray count etc. that are encountered during drilling. These sensors can be employed in a process called ‘measurement while drilling’ (MWD). The data from such sensors are either transferred to a telemetry device, and thence up-hole to the surface, or are recorded in a memory device by ‘logging’.
The sensors and telemetry devices are typically part of a MWD tool, which forms part of a bottom hole assembly (“BHA”) of a drill string that is inserted into a borehole of a well. The MWD tool is housed in a tubular drill collar, and in particular is centered within the collar with enough annular space between the drill collar inner wall and the MWD tool to allow drilling fluid to flow through the annular space to a drilling motor located below the MWD tool and remove drilling cuttings. Some MWD tools are collar mounted, whereas other MWD tools are collar loaded. One or more spiders are used to center and affix a collar loaded MWD tool within the drill collar.
Known spiders comprise an inner rim (“neck”) for coupling to the tool, an outer rim for coupling to the drill collar, and a plurality of spokes that interconnect the inner and outer rims. The spokes extend radially between the rims and are longitudinally aligned in the axial direction. The components of the spider are designed to provide sufficient strength, stability and toughness to withstand the drilling conditions (e.g. temperature, pressure, vibration, shock and erosion) and yet provide sufficient flow area to allow the drilling fluid to flow at or above a minimum flow rate required to power the drilling motor. These competing criteria create a challenge for designers to create effective spiders, particularly for extreme drilling conditions.